In the can industry, coatings containing ZnO have been used for many years to "hide" sulfide staining caused by certain foods during processing and storage cycles (corn, peas, fish, meat products all containing high contents of sulfide proteins). The ZnO usually incorporated at a level between 5%-20% on total coating solids, protects the tinplate (normally more susceptible to staining than tin free steel, also used in can production) by forming white ZnS rather than the more objectionable brown-black tin sulfide.
Traditional solvent-based C-enamels are normally two-package systems consisting of the clear vehicle in one package and the ZnO paste in the other. Zinc oxide is considered a "reactive" pigment. Zinc oxide, and in particular zinc.sup.++ ions have a relatively strong tendency to coordinate or bind with certain ligands and acid or "basic" materials. Zinc oxide can form "zinc soaps" with certain types of resin vehicles with acid functionality. The zinc soap causes a gel-like structure to form in the coating producing an unusable material after a time. The traditional solvent-based C-enamels which used vehicles with low acid value (oleoresinous with low acid, epoxy with no acid, etc. ) have less potential problems with instability than in water-borne systems, which usually contain a high degree of acid functionality.
In water-based C-enamels (those containing zinc oxide pigment) stability has been a major problem resulting in viscosity buildup, pigment or resin settling and loss of flexibility and adhesion. Heretofore, stability of single-package or two-package water-based C-enamel was poor. When the resin vehicle is a typical epoxy-acrylic type with acid number of about 50 to 80 (as described in U.S. Pat. Nos. 4,285,847 and 4,212,781), there is only limited stability. These disadvantages have been overcome by the instant invention which provides a single package-C enamel having excellent stability characteristics.